Conventional fluid dispensing systems include automated and hand-held dispensers that support a barrel containing a liquid to be dispensed. With a hand-held dispenser, an operator orients a dispenser tip at a desired location with respect to a substrate and actuates a switch to initiate a dispensing cycle. A control is responsive to the actuated switch and opens a valve to apply pressurized fluid, for example, shop air, to the rear of a piston in a container or syringe. Thus, as the piston is moved in the container, liquid is dispensed from the tip and deposited on the substrate. Such dispensing systems can dispense a wide range of liquids having a wide range of viscosities, for example, water, resin material, conductive adhesive, cream solder, UV resin, silver paste and the other paste-like materials used in coating, dotting, fixing, filling, sealing and injecting processes used in the manufacture of electronics and many other fields of industry. The quantity of liquid dispensed can be large or small and is often determined by the operator running test dispense cycles of different times and pressures until a desired application of the liquid on the substrate is achieved.
With a container full of liquid, when pressure is applied to piston, the pressure builds rapidly to a desired dispense pressure PD as shown by the pressure-time curve in FIG. 6A; and the pressure is maintained at the desired dispense pressure throughout the dispensing cycle. As liquid is dispensed from the container, the chamber in the container behind the piston increases in size; and hence, a larger volume of air is required to reach the desired dispense pressure. Since air is a highly compressible fluid, a longer time is required to bring the dispenser up to the dispensing pressure; and with the dispensing cycle time fixed, the quantity of liquid dispensed and hence, the size of the liquid deposit, over the dispensing cycle will change. The degree of change will depend on the viscosity of the liquid and other factors. With less liquid in the container, the piston pressure may never reach the desired dispensing pressure as shown in FIG. 6B. Therefore, the operator must continuously adjust the pressure and/or cycle time as the liquid is dispensed from the container.
It is known to use smaller containers, so that the change in volume behind the piston is minimal; and a more consistent dispensing is achieved for a given dispensing pressure and time. However, that solution reduces the efficiency of the dispensing process by requiring more container changes. It also increases the cost of the dispensing process by requiring that more smaller containers be purchased, which also increases the waste generated by the process.
It is also known to automatically adjust the dispensing time in response to liquid being dispensed from the container. In one system, a first calibration is run during which an integration of pressure is determined as liquid is dispensed from the container. A controller stores a running sum of pressures with each time increment, that is, the integrated pressure, which represents the area under the pressure-time curve. Thus, for each time increment, the sum of the pressure integrals represents a quantity of liquid remaining in the container. A second calibration requires that times be measured and stored, which are required to dispense different quantities of liquid from the container. Those stored times also correlate to quantities of liquid remaining in the container. Therefore, over a dispensing cycle, a sum of integrated pressures is determined; and using the results of the first calibration, the control is able to determine the liquid remaining in the container. Based on the second calibration, the control is then able to correspondingly adjust the dispensing time. There are also known variations on this process including variations in the calibration processes as well as having the controller calculate a new dispensing time based on a generic formula.
The above process has several disadvantages. First, it requires multiple calibration processes; and each calibration process requires that a full container of liquid be dispensed until empty before calibration data can be utilized. Thus, the two calibration processes must be executed in an off-line, nonproduction environment, which is a time consuming and costly requirement. Second, finding an integral of the pressure requires that a full dispensing cycle be executed. Thus, any compensation based on the integrated pressure must be implemented on a subsequent dispensing cycle. Third, based on the pressure integral, the duration of a subsequent dispensing cycle is adjusted. While this process may work for manual dispensing processes, it is not practical with automated dispensing processes. In automated processes, a substrate is often moved past the dispenser on a conveyor; and most often, the dispensing process time is considered to be constant, so that conveyor motion past the dispenser can follow a fixed velocity profile. If the dispensing cycle time is constantly changing due to pressure compensation, the velocity profile of the conveyor moving the substrate must also be changed, as well as the timing of the downstream processes operating on the substrate.
Therefore, there is a need to provide an apparatus and a method of providing a simpler and more flexible system for maintaining a consistent quality in the application of the liquid onto the substrate as the liquid in the container is diminished.